1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method. In particular, the present invention relates to an image processing apparatus and an image processing method for forming an image on a recording medium by using a plurality of dot types having different densities per unit area from each other.
2. Description of Related Art
So far, what is available as a printing apparatus for printing an image from a personal computer or the like, is a printer using inks of three colors: cyan (C), magenta (M) and yellow (Y), and a printer using inks of four colors: the above-mentioned three colors and black (K), that is, CMYK. These printers form an image by ON/OFF of dots of each color.
Meanwhile, these days, we can often see a printer performing a printing by forming dots having a plurality of gradation levels in one color, by using inks having different densities from each other in the same color, or by differentiating dot diameters of inks in the same color.
However, although it is possible to form dots having a plurality of gradation levels in one color as mentioned above, an image can be deteriorated when these dots are not properly arranged. For example, when, in order to obtain an output image having a thick dot (dot having a high density per unit area) and a thin dot (dot having a low density per unit area), a halftone process of binarization is applied to the respective dots, and thereafter, the respective pixel values are added to each other, although a dot diffusion of the thick dots only (see FIG. 8B) and a dot diffusion of the thin dots only (see FIG. 8A) are in good conditions, when both the diffusions are overlapped, there are many cases in which the overlapped diffusion cannot be regarded as a good diffusion as a whole. After all, a conclusive granularity is deteriorated to a large degree (see FIG. 8C).
Therefore, various methods with respect to arranging dots of a plurality of dot types having different densities from each other per unit area have been proposed (see JP-Tokukai-2004-1311A, JP-Tokukai-2003-80682A, JP-Tokukai-2000-125121A and Japanese Patent 3208777).
Among these methods, in JP-Tokukai-2004-1311A, a portion in which a plurality of small dots are gathered as a block is replaced with a medium dot, and further a portion in which a plurality of medium dots are gathered as a block is replaced with a large dot.
Further, in JP-Tokukai-2003-80682A, by plotting thin dots at not less than 100%, and when a density difference between the thin dots and thick dots is small, a plotting of thick dots starts.
Further, in JP-Tokukai-125121A, a dot arrangement is made by calculating a dot occupancy of a large dot according to an input value, and thereafter, a three-value halftone process for the input value is performed for allocating one of: plotting a large dot; plotting a small dot; and not plotting a dot.
Further, in Japanese Patent No. 3208777, an input value is broken down into a dot rate between a thick dot and a thin dot, and, in order to have a better thick dot diffusion, a thick dot arrangement is first determined by performing a halftone process to the thick dots, and thereafter, a dot rate of a thin dot is corrected based on the result of the halftone process to the thick dots. Then, a halftone process is performed to the thin dots for determining a thin dot arrangement.
However, in the above-mentioned JP-Tokukai-2004-1311A, a pseudo outline is generated by replacing the dot diameters, and further, when an image is formed of dots having a small dot diameter, a banding due to dot formation unevenness becomes rather noticeable.
Further, in JP-Tokukai-2003-80682A, it is not possible to restrict ink amount, and a type of usable paper is limited.
Further, in JP-Tokukai-2000-125121A, since it is not possible to obtain a favorable image in view of respective dot diffusions, especially a noticeable large dot diffusion, a granularity worsens.
Further, in Japanese Patent No. 3208777, since the thin dot diffusion is disturbed by the thick dots, especially when dot weight is considered, a granularity rather increases. Here, this problem will be described later in detail in an embodiment of the present invention.